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Scary Sounds

What's so awful about nails on a chalkboard?

There are certain sounds that drive us batty. For many people, it's nails on a chalkboard. For me, it's the squeaking sound of styrofoam. What causes this reaction?—Brepark, via the Straight Dope Message Board

I've gotta say, it's a tribute to the unique awfulness of the sound made by nails on a chalkboard that we're even still talking about it—when was the last time you saw a chalkboard, anyway? Back in 2000, dry-erase whiteboards were reportedly outselling the traditional blackboard four-to-one; by now the latter is practically a relic. Here at the Straight Dope, though, we've stayed on the case for 30 years. I first discussed this in a 1986 column where I reported on a study of the "psychoacoustics of a chilling sound": chalkboard scraping. The authors noted that the waveforms of the sound resembled the alarm cries of macaque monkeys and speculated that perhaps our aversive reaction is a vestigial reflex, triggering something in our primate brains alerting us to danger.

I threw a little cold water on this theory back then, and I'm pleased to report the science has caught up with me. In fact, one study from 2003 took the next logical step: polling the monkeys. Not macaques, though; this research looked at cotton-top tamarins, a New World species, comparing their reactions to aversive noises with that of their closest human relatives, Harvard undergraduates. In the experiment, both monkeys and undergrads were exposed to white noise and to a sound "produced by scraping a three-pronged metal garden tool down a pane of glass," described in the paper as a "variant of the fingernails-on-a-blackboard sound"—actual blackboards having already grown scarce in Cambridge, I guess.

Anyways, researchers found that, given the choice to stay in the same spot or move away, the undergrads stayed put when exposed to the white noise but high-tailed it out of there for the scraping. The monkeys, by contrast, didn't seem to care either way. Admitting some drawbacks in the design of the study—notably the use of tamarins, rather than the Old World primates we're more closely related to—the authors concluded that "although such preferences might be innate in humans, they likely have evolved after the divergence point with our primate cousins."

That's what most of the research on this subject is aimed at: innateness. Is our aversion to the sound of nails on a chalkboard—or any number of other commonly detested noises, like your squeaking styrofoam—a learned aversion, or is there something instinctual that causes our discomfort? One 2008 paper out of England sought to determine whether age or gender played a role in how people reacted to a series of "horrible sounds," including our acoustic bete noire, the nails-on-chalkboard sound. (NOC has been the specific focus of most similar research, though you'll be pleased to know that this study's subjects actually rated the styrofoam noise as even more unpleasant.) If aversion is innate, went the reasoning, one might see links to reproductive success: The females of the species would have a stronger negative reaction, given they might be protecting themselves and their offspring, and older folks might have a higher tolerance given their lower procreative potential. The results were suggestive, if only that: Females found NOC to be "slightly worse" than others did, while folks in the 15-35 age range found it "significantly worse" than older or younger people.

Again, intriguing, but clearly to be taken with several grains of salt: The numbers were not only far from conclusive, but they were also obtained via internet survey—meaning factors like speaker quality and playback volume were outside the researchers' control. For a slightly more rigorous analysis, we turn to a 2011 study that attempted to physically quantify reactions to the nails/chalkboard sound. Two European musicologists hooked subjects up to a battery of devices, measuring heart rate, electrical conductivity of the skin, and the like, and let 'er scrape: Participants heard recordings of various sounds, including fingernails and chalk against slate, some modified to exclude certain audio frequencies.

Results? Skin conductivity changed pretty consistently in response to sounds the subjects described as unpleasant, NOC rated foremost among them. The key frequencies for auditory unpleasantness, the data indicated, weren't the high-end ones (and this lines up with the study I looked at in '86) but those between 2,000-4,000 hertz—right in the middle of the range found in human speech. The researchers took this to suggest that the problem might indeed be inbred, if not exactly instinctual: The shape of our ear canals amplifies sounds in that range, meaning we might naturally experience NOC as more intense than sounds at higher or lower frequencies.

There was also evidence pointing to a learned response: Subjects who knew the provenance of the awful noise rated it as more unpleasant than those who were told it came from music. Which, I submit, means we might yet evolve our way out of this situation. Imagine repeating the study with 5-year-olds. Their perception wouldn't be colored by learning where the offensive sound came from. They'd say: What the hell's a chalkboard?